"Breeding-back" aims to restore or immitate extinct animals by selective breeding. This blog provides general information, the facts behind myths and news from various projects.

Tuesday, 7 February 2017

Confirmed: The wisent is an aurochs hybrid

Finally I
have the time to present some interesting news here, that are not that quite
new anymore as the paper appeared in October 2016. But I never found the time
to cover a peer-reviewed paper properly.

In a 2015post, I mentioned the possibility of a hybrid origin for the wisent on my blog
for the first time. I cited a 2004 paper that found the mitochondrial lineage
of wisents to cluster with domestic cattle instead of American bison, although
by Y-data and morphology the wisent is clearly closest to the A. bison. This
provokes the idea that the wisent originated from hybridization of bulls of a
bison-related species with cows of a cattle-related species [1]. Actually, I
recognized some wisent individuals with a horn curvature very reminiscent of
that of the aurochs already in 2013, but was reluctant to propose hybridization
or introgression without having genetic data to back it up.

Wisent with aurochs-like horns at Hellabrunn Zoo, Munich

Now, with
the recent paper by Soubrier et al. being published, the hypothesis of the
wisent being a hybrid of aurochs and steppe bison, Bisonpriscus, seems to
be confirmed [2].

The mystery
of the wisent’s origin starts with the fact that there is no Pleistocene fossil
record of this species, while there is solid evidence for the aurochs or the
Holarctic steppe bison. Now, Soubrier et al. have analysed the genome of specimen
of B. priscus, aurochs, historical
and modern wisents. They found that wisent, aurochs and cattle cluster together
on the mitochondrial genome, while yak, A. bison and Steppe bison form a clade.
Furthermore, they found that a number of Pleistocene remains form a group preliminary
called “Clade X”, which is sister to modern and historical wisents and they
diverged about 120.000 years ago. On the nuclear genome, however, the wisent is
a bison. This implicates that the wisent evolved from hybridization events of
Steppe bison bulls with aurochs cows from more than at least 120.000 years ago (for
comparison: the divergence between taurine and zebuine cattle happened about
250.000 years ago). The polygynical reproduction system of bovines probably
endorsed this asymmetrical hybridization. The hybrids must have been
ecologically different from both ancestor species, as they were reproductively
isolated from then on [2]. This lineage lead to the modern wisent, Bison bonasus. From about 20.000 years
on, wisent-like cave paintings began to show up. Prior to that, they showed a typically
Steppe bison-like morphology.

It is
exciting to have it confirmed, although it does not surprise me at all. Not
only were the occasionally very aurochs-like horns of some wisents suspicious
to me, but also its overall morphology compared to the other fossil and extant species
of Bison. The wisent has a shorter trunk and longer legs, resulting in a
square-like build like in the aurochs, it has a neck bulge and more
horizontally oriented pelvis resembling taurine cattle. Now we can be quite
confident that these traits are not a coincidence, but probably the vestiges of
hybridization with aurochs. After all, both wisent and Clade X have a
proportion of 10,9% of aurochs DNA in their nuclear genome.

Interestingly,
the wisent as a hybrid of two large-horned species ended up being a
short-horned species. To me, there are two explanations for that on genetic
level: either the interplay aurochs and bison genes resulted in short horns,
while respectively having only bison or aurochs alleles on all of these loci
would result in large horns in both species, or new mutations showed up. In any
case, short horns apparently were not disadvantageous to the hybrids, otherwise
they would not have become fixated in the gene pool. Whether this fixation was
the result of selective pressure or genetic drift cannot be said.

Hybridization
is not an uncommon phenomenon. Textbooks tell us that hybrids between wild
species tend to be unsuccessful due to pre- and postzygotic isolation factors
and this might be true for most cases. But it is also evident that
hybridization played a role in the speciation of a number of mammal species,
including wild goats, whales, canines, mice, and our own species, Homo sapiens*. The wisent is yet another
example, and there are probably a lot to find in other vertebrate groups too.

* It has
become well-established by genetic studies of the past several years that
modern Homo sapiens has experienced introgression
by H. neanderthalensis, the Denisova
Man and possible another yet undescribed human species.

The wisent
being an aurochs hybrid of course provokes the question if it might be possible
to obtain nuclear aurochs genes that have vanished from the modern domestic
cattle gene pool (you could expand that even further to the basis of Bos; Banteng and Gaur might share basal
genes that the aurochs also possessed but domestic cattle lost). However, I am
sceptical to this idea. Simply crossing-in wisent in “breeding back” herds and
breeding against obvious bison traits at the maxim “well, it should work
somehow” is probably not a good idea. You would have to know these specific
aurochs genes preserved in wisent and find a way to select on them. It would
probably work only with gene targeting, and with that method you could instead
try to recreate a complete aurochs with the complete genome that already has
been resolved. The influx of wisent into breeding-back herds could also be
problematic for rewilding those herds, as it might increase the tendency of
these cattle to hybridize with wisent in the wild and lower the pre- and
postzygotic isolation barriers; the (not really existent [3]) danger domestic
cattle provide to the genetic integrity of wisent in the wild is sometimes used
as an argument against rewilding wisent and cattle in the same area[3]. So I
would not opt for Frankenstein crossbreeds, also regarding Banteng and Gaur
(the Yak would be the only species for which I am open for experimental
crosses, but that is another story), for practical reasons and public
relations.

An interesting question is if hybridization with bison might have also left a track in the aurochs. There is one detail that makes me consider it not unlikely: the curly hair on the forehead and sometimes also neck and face that we see in many European taurine bulls (and cows, but to a lesser extent). Cattle outside of Europe, especially zebuine cattle, always lack this trait. Perhaps it is a trait that found its way into the gene pool of B. p. primigenius by occasional hybridization with bison and got fixed by sexual selection. But that is only an idea of mine.

" You would have to know these specific aurochs genes preserved in wisent and find a way to select on them. "So it's just the same is with Auroch-genes in domestic cattle.I think it could work to breed some more primiive cattle with some % of european bison in it. Bali catle has some Banteng in it, so why should Bison-cattle crosses have to become Frankensteins ?If one would start with Bison-cows the offsprings would get some rare MtDna. It should be possible to include smaller cattle-breeds like Corriente or Camargue, because of the hybrid vigor of the first generation crosses. Next maybe Moronesa for horn-correction. Last maybe Pajuna. Would this result in Frankensteins ?

No project is actively selecting on specific aurochs genes, but optical traits that are either caused by the same genes as in the aurochs or similar alleles. In the case of the aurochs gene material preserved in wisents, you'd have to actively select on specific genes that are not obviously indicated by optical traits, what makes the whole thing a lot more complicated to practically unfeasible.

Well, they have an ancestor in common. So if there is selection for the hump and the legs maybe it's likely that one would catch up some gene that are related to the Auroch ? Otherwise the ancestor in common should have been humpless and shortlegged...

Of course there is the chance that some of the genes are the same. But that mere chance should not be the basis of a breeding program, especially when it involves hybridization. I would like to have these 10% genes identified regarding position and function before doing anything.

Well, i won't wonder if some influences of european bison could have a grater impact than chianina for example. Something like 1/2 Pajuna, 1/4 Maronesa, 1/8 Corriente and Bison each...But some science wouldn't hurt, of course...

About this blog

This blog is on everything related to the so-called “breeding-back” of extinct animals: From the extinct animals themselves, over their often domestic descendants and dedomestication to news and facts about various breeding-back projects, reports and photos from my own breeding-back related trips. I try to have a balanced and fact-based approach to this subject and to dismantle many of the popular myths. Enjoy!

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About me

My major interest always have been extinct animals, from dinosaurs to Pleistocene megafauna and more recent extinctions. Besides that I am interested in evolution, genetics and ecology.
I am also an amateur animal artist, making drawings and models mostly of extinct animals.